Leniolisib, a selective PI3Kδ inhibitor, is a Food and Drug Administration (FDA)-approved treatment for activated PI3Kδ syndrome (APDS) in patients ≥12 years weighing ≥45 kg. To determine its impact on children, patients aged 4-11 years with APDS were enrolled in a two-part, prospective, open-label, single-arm, international study to evaluate its safety and efficacy (NCT05438407). Results of 12-week treatment (part 1) were previously reported.We report safety and efficacy outcomes of the 1-year extension study. Twenty of 21 patients enrolled in part 1 transitioned to the extension. Leniolisib was administered orally twice daily with dosing based on weight at study visit, ranging from 20–70 mg. The primary endpoint was safety. Additional efficacy outcomes included change from part 1 baseline (CFB) at day (D) 252 in log10-transformed sum of product of diameters (SPD) of index lymph nodes, spleen volume, and naïve B cells. No formal statistical hypotheses were tested.Median treatment exposure was 51 weeks (range, 18–64 weeks). Median treatment compliance was 98.7% (range, 26-100%), with 2 patients ≤80%. Leniolisib was well tolerated (supplemental table). Seventeen patients had 68 adverse events (AEs). The only grade 3 AE was a femur fracture; the rest were grade 1 or 2. Two serious AEs deemed unrelated to leniolisib were reported by 1 patient each: femur fracture and lymphoid tissue operation. Six patients had 7 treatment-related AEs. No AEs led to leniolisib discontinuation. Efficacy outcomes improved at D252 of treatment. Mean CFB in log10-transformed SPD was -0.267 (SD, 0.191; n = 18; 897.1 mm2 to 487.3 mm2), corresponding to -41%. Mean CFB in spleen volume was -66.0 cm3 (SD, 56; n = 20; 242.3 cm3 to 176.3 cm3), corresponding to -25%. Mean CFB in naïve B cells (CD19+CD20+CD27-IgD+) out of total B cells was 9.7% (SD, 9.5; n = 10; 78.6% to 89.7%), corresponding to 27.0% (SD, 26.21; n = 10; 24.9% to 51.9%) in calculated naïve B cells (naïve–transitional B cells) and 25.6% (SD, 19.7; n = 8; 30.6% to 56.2%) in mature naïve B cells.Body weight–adjusted doses of leniolisib were well tolerated and maintained durable outcomes with long-term exposure in patients aged 4-11 years with APDS. Data are consistent with prior studies in adolescents and adults.Tabular data are included as downloadable supplement files.

Background Identifying genetic mechanisms of inborn errors of immunity (IEI) is important for diagnosis and treatment of patients, yet most patients with suspected IEI have negative genetic testing results. Genetic mosaicism is an emerging mechanism of IEI, but it is challenging to identify. Objective We used a discovery-based approach to identify mosaic variants in genes relevant to immune dysregulation in patient and healthy cohorts. Methods We developed custom panels for high-depth sequencing of genes known or hypothesized to cause dominant immune dysregulation. Samples from 452 patients with immune dysregulation (affected) and 154 currently healthy were sequenced using 71- or 101-gene targeted panels. Results We identified mosaic variants in 9.5% of undiagnosed patients and 7.8% of healthy individuals. Using a strategy to predict pathogenicity of variants in IEI, 33% of variants identified in patients were predicted to be likely pathogenic or pathogenic, while no mosaic variants in healthy individuals were predicted to be pathogenic. Genes with mosaicism in >1 affected undiagnosed patients included FAS, STAT3, CARD11, CARD14, NRAS, TNFAIP3, NLRP3, and IKZF2. Four patients had variants in FAS with allele fractions <5% in blood but highly enriched in double-negative T cells, diagnostic for somatic FAS autoimmune lymphoproliferative syndrome. Conclusion These findings establish the utility of a high-depth sequencing panel to identify mosaic variants and demonstrate that mosaicism in immune-relevant genes is present in healthy individuals.

BACKGROUND:Mosaic variants represent a significant but underrecognized contributor to human disease such as cancer and immune diseases. Despite advances in genetic diagnostics, mosaic variant detection remains challenging as a result of low variant allele fractions, tissue specificity, and clinical heterogeneity. OBJECTIVE:We investigated the prevalence, diagnostic impact, and clinical relevance of mosaic variants in participants with immune disorders. METHODS:Exome sequencing of blood and/or saliva was performed in 2655 participants, including 2064 affected participants. Mosaic variants were detected using two algorithms, LoFreq2 and Mutect2. A subset of the detected variants was orthogonally validated. Clinical data were retrospectively analyzed to assess the clinical significance of these variants. RESULTS:Mosaic variants associated with immune disorders contributed to a molecular diagnosis in 29 (1.4%) of 2064 affected participants. Notably, 9 (31%) of 29 of diagnostic variants were missed by standard germline analysis as a result of low variant allele fractions. Clinically relevant parental mosaicism was ascertained in two families. Enrichment of mosaic variants was observed in clonal hematopoiesis-related genes driven by older age and GATA2 deficiency, with prognostic implications for hematologic disorders. Finally, chemotherapy drug resistance variants in NRAS, KRAS, and IDH2 were identified, demonstrating the potential for mosaic variant detection to inform treatment strategies. CONCLUSION:Mosaic variants contribute significantly to the molecular diagnosis and prognosis of immune and hematologic disorders and are missed by typical germline variant-calling workflow.

PurposeActivated phosphoinositide 3-kinase delta (PI3Kδ) syndrome (APDS) is an ultra-rare inborn error of immunity, characterised by immune deficiency and dysregulation. In a randomised controlled trial (RCT; NCT02435173) and open-label extension (OLE; NCT02859727), leniolisib, a selective PI3Kδ inhibitor, was efficacious and well-tolerated in individuals with APDS. These trials suggested some improvements with leniolisib in health-related quality of life (HRQoL) using generic instruments, with other anecdotal evidence also describing improvements in specific domains. Here, all available qualitative data relating to treatment experience were systematically assessed to evaluate any perceived impacts of leniolisib on patients’ lives.MethodsChanges in APDS-related symptoms and patient HRQoL were assessed using unsolicited qualitative data captured from 36 leniolisib-treated individuals: RCT and OLE clinician-recorded, open-text patient narratives (n=31); case reports (n=4); standalone qualitative study conducted with APDS patient/caregiver interviews/narratives (n=1).ResultsImprovements in APDS-related symptoms and HRQoL were reported following treatment with leniolisib: 86.1% (31/36) of individuals mentioned improvements in ≥1 symptom/HRQoL impact. Of these, 87.1% (27/31) explicitly attributed ≥1 improvement to leniolisib. One-third (12/36) of individuals explicitly attributed improvements in fatigue/energy to leniolisib. This was associated with reported HRQoL improvements explicitly attributed to leniolisib: physical activity (33.3% [12/36]), work/school performance/attendance (13.9% [5/36]) and travel ability (8.3% [3/36]). Moreover, 36.1% (13/36) of participants explicitly attributed improvements in their overall wellbeing to leniolisib.ConclusionAs data were unsolicited and baseline symptoms/HRQoL were not available, results may not represent all changes experienced, and is not possible to determine the amount of change in symptoms/HRQoL associated with leniolisib. Nevertheless, these analyses indicate that participants with APDS experienced improvements in symptoms and overall wellbeing following treatment with leniolisib.

TLR8 gain-of-function (GOF) somatic variants were recently identified as causing severe neutropenia, lymphoproliferation, and immune dysregulation. We report the expanded clinical and laboratory phenotype and management of 10 patients, including the original cohort of 6 male patients. We identify the first female patient with TLR8 GOF who presented during infancy with pure red cell aplasia due to a germline TLR8 variant, and two new disease-causing somatic variants in male patients. Eight patients had somatic mosaicism with peripheral blood variant allele fractions of 7-26% and age of disease onset of 9 months to 28 years. All patients had neutropenia, most with severe neutropenia refractory to medical therapy. Anemia and thrombocytopenia were common. Bone marrow characteristically demonstrated severe myeloid hypoplasia and activated T-cell infiltrates and/or aggregates. An increased number of large granular lymphocytes (LGLs) were identified in five patients. Seven patients underwent allogeneic hematopoietic cell transplantation (HCT). High rates of post-HCT cytopenia of unclear etiology and graft-versus-host disease were observed. Five are surviving at 1 to 3 years post-HCT with full donor myeloid and T-cell chimerism and resolution of disease phenotype. The two patients who presented during childhood and did not undergo HCT ultimately died from disease. In conclusion, TLR8 GOF is an X-linked dominant disorder that should be considered in male and female patients with cytopenia, particularly severe neutropenia, lymphoproliferation with immune dysregulation, increased LGLs, and new to this cohort, red cell aplasia. Disease is refractory to medical management, and curative, allogeneic HCT should be considered early after diagnosis. (NCT04339777)

Activated phosphoinositide 3-kinase delta syndrome (APDS) is an inborn error of immunity characterized by a combination of immunodeficiency with immune dysregulation and classified within autoimmune lymphoproliferative immunodeficiencies. Early therapeutic strategies extrapolated from autoimmune lymphoproliferative syndrome, including mechanistic target of rapamycin (mTOR) inhibition with sirolimus, provided variable control of lymphoproliferation. Recognition of constitutive phosphoinositide 3-kinase δ (PI3Kδ) signaling led to the development of leniolisib, approved by the US Food and Drug Administration in 2023, marking a transition toward pathway-specific precision therapy. In this Clinical Perspective, we review the evolution of targeted therapy in APDS, the biochemical structure and selectivity of leniolisib relative to other PI3K inhibitors, and the mechanistic distinction between proximal PI3Kδ inhibition and downstream mTOR blockade. We summarize current clinical evidence and practical considerations for dosing and monitoring in the absence of serum drug-level assays. Finally, we outline key unresolved questions, including long-term disease modification, biomarker validation, pediatric outcomes, treatment sequencing and combination strategies, while positioning leniolisib relative to hematopoietic stem cell transplantation, and desirability of its wider global access.

Heterozygous NFKB1 variants have been increasingly identified in patients with common variable immunodeficiency. These patients, however, often experience substantial immune dysregulation beyond humoral deficiency. NFKB1 encodes p105, which undergoes proteolytic cleavage to release p50, the transcriptionally active subunit of canonical NF-κB. While several in vitro transfection systems have demonstrated loss-of-function of these variants regarding canonical NF-κB reporter activity, the immune-related phenotypes of these patients differ from those with well-characterized defects leading to impaired canonical NF-κB activation (i.e., IKBKG or NFKBIA variants). Notably, the C-terminal part of p105 contains ankyrin repeats, which can repress downstream signaling by retaining NF-κB dimers in the cytosol. We therefore hypothesize that loss of p105 inhibitory function may contribute to the immune dysregulation observed in some affected patients.In this study, we evaluated 8 patients from 8 unrelated kindreds presenting with early-onset common variable immunodeficiency, complicated by diverse and severe immune dysregulations, including multiple autoimmunity and lymphoproliferation (NCT00001355). In addition to immunoglobulin replacement therapy for hypogammaglobulinemia, all required multiple immunomodulatory therapies. One patient ultimately underwent allogeneic hematopoietic stem cell transplantation for disease control.All patients harbored private germline heterozygous NFKB1 truncating variants (nonsense, n = 2; frameshift, n = 5; splice, n = 1) located in the N-terminal p50 part of the protein, prior to the ankyrin repeats. Beyond the well-recognized B cell failure in these patients, peripheral blood mononuclear cells showed lower basal IκBα level and enhanced IκBα degradation following ex vitro stimulation, indicating activation of canonical NF-κB signaling. Their T cells showed increased phospho-AKT upon stimulation. Together, these findings suggest a potential loss of p105 inhibitory function in these patients. Further elucidation of the mechanisms underlying profound immune dysregulation is crucial because these patients may benefit from the PI3Kδ inhibitor leniolisib, given the observed PI3K signaling activation. A prospective clinical trial is currently recruiting such participants to investigate this targeted therapy (NCT06549114).

Leniolisib is an FDA-approved PI3Kδ inhibitor used to treat activated phosphoinositide 3-kinase delta syndrome (APDS) in patients aged ≥12 years who weigh ≥45 kg. Pediatric patients aged 4 to 11 years with APDS were enrolled in a two-part, prospective, open-label, single-arm, international study to evaluate the safety and efficacy of leniolisib (NCT05438407). Here we report safety and efficacy outcomes following the completion of 12 weeks of leniolisib treatment (part 1). Twenty-six patients were screened; 21 patients enrolled across the United States, France, and Japan (Table 1). Leniolisib was administered orally twice daily (BID) as tablets in 10- and 30-mg strengths, with doses ranging from 20 to 70 mg BID based on weight. Co-primary outcomes were change from baseline (CFB) at 12 weeks in log10-transformed sum of product of diameters (SPD) of index lymph nodes and normalization of naïve B cells. Additional outcomes included changes in spleen volume and quantitative immunoglobulin (Ig) levels, including IgM. Table 1.Patient demographics and baseline characteristics.Patient Characteristic (N=21)ValueAge at time of study, median (range), y7.0 (4-11)Sex, male to female, n13:8Self-identified race, n (%)White9 (42.9)Asian4 (19.0)Other1 (4.8)Not reported/unknown/missing7 (33.3)Self-identified ethnicity, n (%)Hispanic3 (14.3)Non-Hispanic12 (57.1)Not reported6 (28.6)Weight at baseline, median (range), kg23.9 (15.4-44.5)Time since APDS diagnosis, median (range), mo32.9 (2.2-142.2)APDS variant distribution, n (%)PIK3CD17 (81.0)PIK3R14 (19.0)APDS, activated phosphoinositide 3-kinase delta syndrome; PIK3CD, phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit delta; PIK3R1, phosphoinositide-3-kinase regulatory subunit 1. Both primary end points improved from baseline after 12 weeks of treatment with leniolisib across all dose levels. Mean CFB of log10-transformed SPD reduced by -0.1956 (n = 19). Mean CFB of naïve B cells (CD19+CD27-CD10-) from total B cells was 33.3% (SD, 13.1; n = 11). Mean CFB of spleen log10 volume reduced was -0.1222 (n = 21). Mean Ig levels (n = 14) changed from baseline after 12 weeks of leniolisib treatment: IgM, 2.7 g/L to 1.6 g/L; IgG, 10.1 g/L to 11.1 g/L; and IgA, 0.88 g/L to 0.83 g/L. Leniolisib was well tolerated (Table 2). Twenty patients had treatment-emergent adverse events (AEs). All were grades 1 or 2; none were serious. Five patients had treatment-related AEs, including headache, palpitations, abdominal pain, diarrhea, nausea, pruritus, fatigue, decreased neutrophil count, and increased aspartate aminotransferase. No AEs led to discontinuation of study drug treatment. Table 2.Summary of Treatment-Emergent Adverse Events.Adverse Events CategoryTotal No. of Patients, n (%)Any TEAE20 (95.2)Grade 120 (95.2)Grade 28 (38.1)Grade 30Grade 40Grade 50Any study treatment-related TEAE5 (23.8)Leading to discontinuation of study treatment0Leading to death0Any serious TEAE0TEAE, treatment-emergent adverse event. Overall, leniolisib was well tolerated, reduced lymphoproliferation, and improved normalization of naïve B cells, meeting the co-primary end points. All 21 patients completed part 1, and 20 transitioned to the 1-year treatment extension (one patient discontinued due to social reasons).

Activated phosphoinositide 3-kinase delta (PI3KS) syndrome (APDS) is an ultra-rare, progressive genetic disease, characterised by immune deficiency and dysregulation, affecting individuals from birth. In a 12-week phase III randomised placebo-controlled trial, leniolisib, a selective PI3KS inhibitor, was well-tolerated and met both co- primary endpoints (change from Baseline in log10-transformed sum of product of diameters of index lymph nodes and percentage of na & iuml;ve/total B cells at Day 85). Here, prespecified subgroup analyses are reported in adolescents aged 12-17 years (leniolisib, n = 8; placebo, n = 4) and adults aged >= 18 (leniolisib, n = 13; placebo, n = 6). In both subgroups, leniolisib reduced lymphadenopathy (least squares mean change versus placebo: adolescents,-0.4 versus-0.1; adults,-0.3 versus 0.1) and increased the percentage of na & iuml;ve B cells (least squares mean change: adolescents, 44.5 versus-16.5; adults, 28.4 versus-1.1). Leniolisib was well-tolerated in both adolescents and adults. These results show leniolisib is an effective APDS treatment in both subpopulations. Plain Language Summary: What is activated PI3K delta syndrome (APDS)? APDS is an ultra-rare disease in which the immune system does not work correctly. People with APDS have a wide range of symptoms, including infections, certain organs associated with the immune system becoming larger, and worse quality of life. These symptoms generally start in childhood. Why was this study carried out?

BACKGROUND:Hematopoietic cell transplantation (HCT) provides effective long-term management for some inborn errors of immunity. Genetic findings can inform donor selection, considerations in conditioning intensity and agents, and graft-versus-host disease prophylaxis. Exome/genome sequencing is increasingly accessible but of uncertain clinical utility. We aimed to evaluate the clinical utility of comprehensive genomic evaluations through review of HCT at our center. METHODS:We performed exome/genome sequencing on pre-HCT samples from participants between 2017 and 2023. We reported primary findings (PF) and secondary findings (SF). Post hoc, we analyzed medication and pharmacogenetic (PGx) data. RESULTS:We analyzed pre-HCT exome/genome sequencing (n = 84 exome, n = 63 genome, n = 32 with both) for 179 probands. Most (143/179; 79.9%) had a PF underlying the HCT indication, with GATA2 being most common (n = 59). Three percent of participants had an SF predisposing to cancer or cardiovascular disease. Most (n = 108/179; 60.3%) received ≥1 medication(s) that may have been further optimized with PGx. Using Kaplan-Meier survival analysis, we compared the survival rates of participants with 0, 1, and ≥2 genomic risk factors (GRF: absence of PF; presence of SF or PGx). Survival at 3 y was 94.8%, 84.8%, and 58.5% for those with 0, 1, and ≥2 GRF, respectively (log-rank: 16.10, df = 2, P  = 0.0003), indicating statistically significant survival differences by GRF. CONCLUSIONS:Comprehensive genomic evaluation is an emerging avenue for tailoring HCT approaches, and identification of HCT-relevant findings may be common. On multivariate analysis, GRF was associated with survival in this retrospective cohort. Prospective research is warranted to further integrate genomic data into precision treatment.

BACKGROUND:Despite suppressive antiretroviral therapy (ART), 15%-30% of people with human immunodeficiency virus (HIV) experience a limited recovery of CD4 T cells. Although autoantibodies against the CD4 receptor have previously been identified in people with HIV (PWH), little is known about their longitudinal impact on CD4 T-cell reconstitution. METHODS:Anti-CD4 autoantibodies were evaluated by the fluid-phase luciferase immunoprecipitation systems immunoassay in ART-naive people with advanced HIV (CD4 count ≤100 cells/µL), PWH with CD4 count >200 cells/µL, long-term nonprogressors, people with idiopathic CD4 lymphopenia, people with autoimmune lymphoproliferative syndrome, and healthy volunteers without HIV. In the participants with advanced HIV, we assessed the association of anti-CD4 autoantibodies at ART initiation with CD4 recovery over a median follow-up of 192 weeks. RESULTS:Anti-CD4 autoantibodies were identified in 29% (61/210) of ART-naive participants with advanced HIV but were absent in people without HIV. Female PWH showed a 4-fold higher prevalence (P < .001) of anti-CD4 autoantibodies compared to males. After ART initiation, people with advanced HIV with anti-CD4 autoantibodies exhibited an overall slower rate of CD4 reconstitution (5.8 vs 6.6 cells/µL/month, P = .007) and lower week 192 CD4 count (268 vs 355 cells/µL, P = .037). Incidental, clinically indicated immunosuppressive therapy in these participants was associated with an improved rate of CD4 reconstitution (P = .0019) and higher week 192 CD4 count (551 vs 268 cells/µL, P = .019). CONCLUSIONS:People with advanced HIV harboring anti-CD4 autoantibodies at ART initiation demonstrated a slower rate and extent of CD4 reconstitution after 4 years. Incidental immunosuppressive therapy was associated with increased CD4 counts in these participants.

Chronic norovirus infection (CNI) is a significant cause of morbidity and mortality in immunocompromised patients. We sought to determine clinical and virologic features associated with CNI with an emphasis on IEI. Norovirus-positive stool samples from 88 patients at the NIH Clinical Center from 2010 to 2023. Longitudinal clinical and virologic data were collected. Forty-eight patients had CNI; 23 had acute norovirus infection (ANI); 17 patients were unclassifiable. Eighty-three percent of CNI patients had an underlying diagnosis of IEI, including CID (N = 23), CVID (N = 11), and SCID (N = 6). The majority with ANI had hematologic disorders (48%); only 5 (21%) had CVID, CID, or SCID. Mortality in the CNI cohort was 38%. More than half of CNI patients had nutritional defects, including electrolyte abnormalities and hypovitaminosis; 20% required TPN. GII.4 was the predominant norovirus genotype among all patients, although diverse genotype I and II viruses were represented. Gastrointestinal coinfections during CNI infection were prevalent (50%); the most common being enteropathogenic E. coli (N = 12) and C. difficile (N = 12). Seventy-seven percent of CNI patients had chronic liver disease, including both cholestatic (N = 18) and hepatocellular (N = 14) patterns. Thirty-three percent (N = 16) of CNI patients had a diagnosis of nodular regenerative hyperplasia (NRH). Only 35% of patients with ANI had chronic liver disease. Twelve patients with CNI eventually cleared norovirus (median duration of 688 days versus 18 days for ANI). Eight patients cleared in the setting of definitive immune reconstitution, specifically HSCT or gene therapy. Three patients cleared in the setting of immunomodulatory therapy for concurrent IEI-related enteropathy treated with ustekinumab, abatacept, and/or JAK inhibitors. No patient cleared after traditional CNI therapies, including nitazoxanide, ribavirin, or immunoglobulin (PO or IV). CNI patients were nutritionally compromised with high mortality and a high prevalence of liver disease. IEI with adaptive defects were disproportionately represented in CNI. Common CNI treatments were not effective. CNI consistently resolved with definitive immune reconstitution; some patients cleared infection with immunomodulatory therapy for enteropathy, suggesting a new treatment strategy to be explored in future studies. To our knowledge, this is the largest cohort of CNI with a focus on IEI analyzed to date. Figure 1. Category of underlying diagnosis. Figure 2. Norovirus genotypes represented across cohorts.

Autoimmune lymphoproliferative syndrome (ALPS) is an inborn error of immunity (IEI) characterized by abnormal FAS-mediated apoptosis of lymphocytes that leads to lymphoproliferation and expansion of CD4−/CD8− double-negative T cells (DNTs). In patients with ALPS-FAS, DNTs have been reported to exhibit increased activity in the phosphoinositide 3-kinase δ (PI3Kδ)/mammalian target of rapamycin (mTOR) pathway. Although mTOR inhibition with sirolimus has improved autoimmune cytopenias and organomegaly in patients with ALPS, it requires monitoring of serum levels, and common adverse events frequently hamper long-term use. As leniolisib, a selective PI3Kδ inhibitor, reduced lymphoproliferation in another IEI known as activated PI3Kδ syndrome, efficacy was examined in a murine model of ALPS. Changes in organ weight and key immune subsets in MRL/lpr−/− mice receiving vehicle or leniolisib (40 or 80 mg/kg/day) by oral gavage were assessed. Leniolisib limited the canonical features of ALPS, including lymphadenopathy, splenomegaly, and elevated DNTs, in a dose-dependent manner. These results support the evaluation of leniolisib in patients with ALPS (NCT06549114).

Somatic mosaicism is an important mechanism of immune dysregulation and explains a growing proportion of the 70% of patients with suspected IEI who lack a molecular diagnosis. Identifying disease-causing somatic mutations is challenging due to the presence of such variants in a small subset of cells, leading to a low variant allele fraction (VAF). We developed a custom 71-gene capture-based panel for high-depth sequencing of genes known or hypothesized to be associated with dominant immune dysregulation. Sequencing was performed with samples from 223 patients with immune dysregulation phenotypes and 96 currently healthy individuals. Mean age was 24.5 years for patients and 17.8 years for healthy individuals. 52.5% of patients and 40.6% of healthy individuals were biologic males. Coverage across coding regions of targeted genes had an average depth of 5752×. Our custom analysis pipeline utilized the union of multiple variant callers to call somatic variants. We identified 36 somatic variants in 21 genes from 28 patients (12.6% of patients) and 6 healthy controls (6.25% of healthy). To validate our assay, an independent library was sequenced for 60 patients and 18 healthy controls, including samples harboring 27 somatic variants. Droplet digital PCR (ddPCR) was used as an orthogonal method to validate 17 variants. All tested variants were confirmed as somatic. Genes with somatic mosaicism in >1 affected patient included FAS, STAT3, CARD11, NRAS, TNFAIP3, NLRP3, and PIK3CD. Three variants were previously described to cause somatic immune dysregulation. Immune cell populations from 7 patients with variants in FAS, NRAS, TNFAIP3, and UBA1 were purified and ddPCR performed, which identified enrichment of somatic variants in different cell types. This included somatic variants in FAS that had a VAF of 5-7% in whole blood but 21-28% in DNTs, all in individuals with a clinical phenotype consistent with ALPS. Our studies demonstrate the utility of high-depth targeted sequencing of suspected IEI patients to identify pathogenic and potentially pathogenic somatic variants, including low VAF variants directly from whole blood. Ongoing studies include expanding our gene panel, in vitro functional validation of novel variants, and utilizing single-cell technologies to identify the functional impact of somatic variants.

Treating patients with ALPS can be challenging. This article examines the efficacy of leniolisib, a selective PI3Kδ inhibitor, in a murine model of ALPS, illustrating a reduction in lymphoproliferation and disease-associated biomarkers such as elevated double-negative T cells.

Refractory autoimmune mutilineage cytopenias can present in childhood associated with chronic nonmalignant lymphoproliferation (splenomegaly, hepatomegaly, and/or lymphadenopathy). Cytopenias due to peripheral destruction and sequestration have been well recognized since the 1950s and are often lumped together as eponymous syndromes, such as Evans syndrome and Canale-Smith syndrome. Though their clinical and genetic diagnostic workup may appear daunting, it can provide the basis for early intervention, genetic counseling, and empirical and targeted therapies. Auto- immune lymphoproliferative syndrome (ALPS), activated phosphatidylinositol 3-kinase delta syndrome (APDS), and many other related genetic disorders are otherwise collectively known as inborn errors of immunity (IEI). They present in early childhood as refractory autoimmune cytopenias due to immune dysregulation leading to lymphadenopathy, splenomeg- aly, and increased susceptibility to lymphoma. More recently, controlled clinical trials have shown that some of these immune system disorders with hematological manifestations might be more readily amenable to specific targeted treatments, thus preventing end-organ damage and associated comorbidities. Over the last 20 years, both rapamycin and mycophenolate mofetil have been successfully used as steroid-sparing long-term measures in ALPS. Current therapeutic options for APDS/PASLI (phosphoinositide 3-kinase [PI3K]-associated senescent T lymphocytes, lymphadenopathy, and immunodeficiency) include the orally bioavailable PI3K delta inhibitor, leniolisib, which was licensed by the US Food and Drug Administration (FDA) in 2023 for use in individuals older than 12 years as a targeted treatment. Paradigms learned from patients with rare genetic disorders like ALPS and APDS may help in exploring and streamlining molecular therapy strategies in the wider group of IEIs presenting with refractory cytopenias and lymphoproliferation.

The differentiation of naive and memory B cells into antibody-secreting cells (ASCs) is a key feature of adaptive immunity. The requirement for phosphoinositide 3-kinase-delta (PI3K delta) to support B cell biology has been investigated intensively; however, specific functions of the related phosphoinositide 3-kinase-gamma (PI3K gamma) complex in B lineage cells have not. In the present study, we report that PI3K gamma promotes robust antibody responses induced by T cell-dependent antigens. The inborn error of immunity caused by human deficiency in PI3K gamma results in broad humoral defects, prompting our investigation of roles for this kinase in antibody responses. Using mouse immunization models, we found that PI3K gamma functions cell intrinsically within activated B cells in a kinase activity-dependent manner to transduce signals required for the transcriptional program supporting differentiation of ASCs. Furthermore, ASC fate choice coincides with upregulation of PIK3CG expression and is impaired in the context of PI3K gamma disruption in naive B cells on in vitro CD40-/cytokine-driven activation, in memory B cells on toll-like receptor activation, or in human tonsillar organoids. Taken together, our study uncovers a fundamental role for PI3K gamma in supporting humoral immunity by integrating signals instructing commitment to the ASC fate. Lucas and colleagues show that differentiating B cells switch expression of PI3K delta to PI3K gamma and that this switch is required for optimal T cell-dependent IgG antibody production in both mice and humans.